1. Field of the Invention
The invention relates to a control device for an internal combustion engine and, more particularly, to a control device for an internal combustion engine equipped with a variable valve mechanism that variably sets the valve timing.
2. Description of the Related Art
A control device for an internal combustion engine equipped with a variable valve mechanism is known, as disclosed in, for example, Japanese Patent Application Publication No. 2001-355462 (JP-A-2001-355462). This kind of control device for internal combustion engines according to the related art has a construction in which an exhaust valve early-closure control is executed when an internal combustion engine is undergoing a low-speed and low-load operation.
In the exhaust valve early-closure control, the exhaust valve closure timing is advanced by operating a variable valve mechanism. As a result, in the vicinity of the intake top dead center, a period during which the intake and exhaust valves are both in a closed valve state (so-called minus overlap period) occurs, or an existing minus overlap period is elongated.
During the minus overlap period, the exhaust gas remaining in each cylinder is compressed to have high temperature by an ascending motion of the piston. Therefore, in the related art, the combustion characteristic is bettered by producing a high-temperature remaining exhaust gas through the exhaust valve early-closure control, and by utilizing the remaining exhaust gas as an internal EGR gas.
Besides, in the related art, when the idle operation is performed, the exhaust valve early-closure control is not executed, but the exhaust valve is closed substantially at the intake top dead center. As a result, the combustion characteristic becomes stabilized by an amount that corresponds to the reduction (or elimination) of the blowback of exhaust gas. In the related art, therefore, the idle stability is improved.
By the way, in the related art, the remaining exhaust gas in each cylinder is compressed during the minus overlap period by the exhaust valve early-closure control. In the exhaust valve early-closure control, if the intake valve opens after the minus overlap period ends, there occurs a blowback of the remaining exhaust gas from the cylinder to the intake system.
Hence, if the exhaust valve early-closure control is implemented during a period from the starting of the internal combustion engine until after the engine has been started up, the state of combustion can be bettered by the blowback of exhaust gas. That is, when the intake negative pressure is small at the time of and after the starting of the engine, the blowback of exhaust gas is caused to adequately occur, and is caused to collide with injected fuel so that the fuel is atomized into fine particles.
However, for example, at the time of the first idle operation immediately following the startup of the engine, the intake negative pressure increases with rises in the engine rotation speed. If the exhaust valve early-closure control is implemented in a state in which the intake negative pressure has increased in the foregoing manner, the amount of blowback of exhaust gas is likely to become excessively large and the combustion state is likely to deteriorate since the pressure difference between the inside of the cylinder and the intake side is great. Therefore, the related art has a problem of it being difficult to favorably carry out both the early-closure control and the idle stabilization even if the exhaust valve early-closure control can be implemented at the time of idle operation.